The invention relates generally to nondestructive ultrasonic pulse-echo inspection of workpieces, and particularly to an improved lens for directing the ultrasonic energy into a workpiece.
The nondestructive ultrasonic inspection of a workpiece is a well-developed technique. Briefly, in a typical inspection set-up for flat or curved plate, the workpiece to be inspected and a source of ultrasonic energy (usually a transducer and lens), are placed in close proximity to one another and may be joined by a coupling fluid such as water. Energy from the lens is directed from a sensing wafer through the lens toward the workpiece, refracted at the workpiece surface from longitudinal into shear mode and reflected inside the workpiece back to the sensing wafer. Variations in reflected energy received by the sensor indicate presence of a crack or flaw in the workpiece.
Heretofore in a typical ultrasonic inspection apparatus, a flat lens directs ultrasonic energy through a coupling medium, e.g. water into the workpiece. Either the transducer or the workpiece moves so that the width of ultrasonic energy sweeps through the workpiece. Often, shear waves are used to detect cracks. Shear waves are created inside the workpiece by tilting the transducer relative to the surface of the workpiece. Ideally, the ultrasonic energy should be concentrated in a straight line parallel to the surface of the workpiece. However, the tilt inclines the focal zone (far field) of energy so that it is not parallel to the surface. This is undesirable in that it limits the width of the scan path and prevents uniform accuracy of flaw detection over the width of the scan path.
In the present invention, there is a novel cylindrical or flat lens having curved or straight multiple wedge-shaped elements that focus the ultrasonic energy in a shear mode substantially in a straight line parallel to the surface within a workpiece. The advantages of this arrangement are (1) errors in detection are minimized by maintaining a constant measurement accuracy over the entire width of the scan; (2) wide transducers can be used to generate wide paths of scan; (3) flaws can be accurately located as to depth within the workpiece; and (4) the lens of the present invention is the only known shape of lens that can achieve the above three advantages on circumferential cracks in tubular shaped products as well as on cracks oriented parallel to scan movement in flat shaped workpieces.